Security features are used to reduce or prevent counterfeiting of items. These items may have a high intrinsic value, such as banknotes, or they may be critical parts in other items, such as brake pads in fighter planes. By tagging an item with a security feature, the authenticity of the item can be verified by validating that the security feature is genuine.
Many different types of security feature are available. One type is an optically detectable security feature comprising silica particles doped with rare earth ions.
Rare earth ions are known to have certain photoluminescent properties that make them useful in security features. In particular, when a rare earth ion is excited it emits a fluorescent fingerprint, in the form of well defined and relatively narrow, non-overlapping spectral peaks. However, due to the small size of security features used (less than five micron diameter), it is difficult to generate sufficient luminescence by excitation of the rare earth ion, because relatively few photons are absorbed by the rare earth ions.
U.S. Pat. No. 7,129,506 to Ross et al. discloses a unique optically detectable marker that comprises a glass carrier incorporating a rare earth ion, which overcomes the rare earth ion's limited ability to absorb light. When the rare earth dopant is incorporated into the glass, new bonds are formed in the doped glass, thus altering the electron arrangement and hence the energy levels of absorption and fluorescent emission.
The security features disclosed in Ross et al. suffer certain limitations. For example, the security features disclosed by Ross et al cannot easily, consistently, and uniformly be made any smaller than approximately five microns. The difficulty in making such security features smaller than five microns excludes the use of the markers in certain applications.
Furthermore, the security features in Ross et al are manufactured using a cast and smash technique that involves casting an ingot of rare earth doped glass and then breaking the ingot into small pieces for use as a security feature.